Many physical implementations of quantum information systems have been proposed. One class of quantum system uses the quantum states of atoms, ions, molecules, or other matter to represent quantum information such as qubits. Some particularly promising proposals of this type implement unitary operations and readout in matter qubits via laser-driven optical transitions. Examples of these systems include ion-trap systems such as described by J. Cirac and P. Zoller, Phys. Rev. Lett. 74, 4091 (1995), system using nitrogen-vacancy (N-V) defects in diamond as described by Jelezko et al., Phys. Rev. Lett. 92, 076401 (2004) and Nizovtsev et al., Optics and Spectroscopy 99, 233 (2005), and systems using the Pauli-blockade effect in quantum dots with a single excess electron as described by Pazy et al., Europhys. Lett 62, 175 (2003) or by Nazir et al., Phys. Rev. Lett. 93, 150502 (2004).
Other proposed quantum information systems use the quantum states of photons to represent qubits. Knill, Laflamme, and Milburn, Nature 409, 26 (2001), for example, proposed a quantum computing system using optical qubits manipulated with linear optics. Optical quantum information systems using non-linear light-light interactions such as provided in some systems exhibiting electromagnetically induced transparency (EIT) have also been developed. See, for example, R. G. Beausoleil, W. J. Munro, and T. P. Spiller, Journal of Modem Optics 51, 1559 (2004).
Matter and photonic quantum information systems each have advantages. For example, stationary matter systems may be better for storage of quantum information than are photons, which inherently move. In contrast, implementing quantum interactions between separated matter systems can present technological and architectural challenges, especially in systems containing many qubits. In contrast, optical systems capable of selecting and bring photons together for interactions may be constructed using conventional optical systems. A hybrid system using both matter and photons to represent or convey quantum information may be able to use the advantages of matter and photonic systems. However, such hybrid systems will generally require structures and techniques for converting or transferring quantum information between matter and photonic representations.